Patent Document 1 describes a technique in which, when a proportion of nitrogen dioxide (NO2) in nitrogen oxides (NOx) contained in exhaust gas (hereinafter, referred to as a “NO2 proportion”) is greater than a desired proportion in a configuration in which an exhaust gas purification apparatus including a selective catalytic reduction catalyst (SCR catalyst) is arranged in an exhaust passage of an internal combustion engine, the NO2 proportion is reduced by reducing an amount of exhaust gas recirculation (EGR) gas.
Patent Document 2 describes that a NO2 proportion in exhaust gas increases when an air-fuel ratio of an air-fuel mixture burned in a cylinder of an internal combustion engine is high (lean) as compared to when the air-fuel ratio is low (rich).
Patent Document 3 describes a technique for controlling any one of or both an EGR rate and a fuel injection timing so as to set a NO2 proportion to approximately ½ (50 percent) in a configuration in which an exhaust gas purification apparatus including an SCR catalyst is arranged in an exhaust passage of an internal combustion engine.